The present invention relates generally to a silencer that reduces noises from a noise source, and an electronic apparatus having the same. The present invention is suitable, for example, for a silencer that reduces noises from a cooling fan for cooling a CPU in an electronic apparatus such as a personal computer (“PC”).
Along with the recent widespread electronic apparatuses, a high-performance, high-environmental-performance and small PCs have been increasingly demanded. For high-performance actions, both the number of exoergic circuit elements, such as CPUs, and each exoergic element's calorific value are likely to increase. Since the heat may damage the exoergic element and surrounding circuit elements or cause their operational failures, a cooling system that uses a cooling fan for cooling is conventionally proposed. The term “cooling” in this specification is a concept that covers heat radiations.
However, this cooling fan becomes a noise source and aggravates the environmental performance. Therefore, a PC having a silencer that reduces noises is proposed. One illustration is to provide a silencer to a duct in the cooling system that houses a cooling fan in the duct and sends winds to and cools a downstream exoergic element in the duct. See, for example, PCT International Publication No. WO04/061817. The duct serves as a housing that defines a space that encloses a noise source, such as the cooling fan, and the silencer serves to reduce noises in the space or at the exit of the space.
FIG. 10 is a schematic perspective view of a silencer for explaining a typical sound attenuation theory. FIG. 11 is a frequency characteristic of the silencer's sound damping quantity. See, for example, FIG. 1 of PCT International Publication No. WO 04/061817.
Other prior art include, for example, Japanese Patent Application, Publication No. 2001-222065.
According to Aritomo Nakano, “Ultrasonic (inaudible sound)—fundamental, measurement, evaluation, reduction measures,” Gijyutu Shoin, Jun. 20, 1994, p. 98, a frequency f0 of a sound-attenuated target is theoretically given by the following equation:
                              f          0                =                              c                          2              ⁢                                                          ⁢              π                                ⁢                                                    c                0                            V                                                          EQUATION        ⁢                                  ⁢        1            
S1 is a tube's sectional area (m2), V is a cavity's capacity (m3), c0 is an electrical conductivity (S/m), S0 is a pore's area (m2), c0=S0/le, le is a tube end correction length (m), n is the number of pores, d is a pore's diameter (m), and c0=nS0/(t+0.8vS0). When t is sufficiently small, c0˜d.
A theoretical equation of the sound damping quantity (transmission loss) is given as follows:
                    TL        =                  10          ⁢                                          ⁢          log          ⁢                      {                          1              +                                                (                                                                                                                                                                        c                              0                                                        ⁢                            V                                                                          /                        2                                            ⁢                                              S                        1                                                                                                            f                        /                                                  f                          0                                                                    -                                                                        f                          0                                                /                        f                                                                              )                                2                                      }                                              EQUATION        ⁢                                  ⁢        2            
From Equations 1 and 2, as the cavity's capacity V of the silencer increases, the low-frequency sound can be deadened and the sound damping quantity becomes large. The noise of the cooling fan is the low-frequency sound between about 500 Hz and about 1000 Hz, and it is thus necessary to increase the capacity of the silencer housed in the duct for effective sound deadening. However, as the silencer's capacity increases, the duct and the PC's housing enlarge contrary to a demand for the miniaturization.